Monitoring spots via the TESS data strengthened by the ETV behaviour: An investigation on the contact binary PP Lac

In this study we aim to reveal the starspot behaviour in the contact binary PP Lac, using the clues from the eclipse timing variation (ETV) nature of the system, obtained from the TESS data. In this context, we performed simultaneous ground-based light and radial velocity curves analysis and calculated the absolute physical and orbital parameters of the system. These parameters were then used during the light curve analysis of the TESS data for a detailed spot modelling. We considered three different spot models with: a single spot, two overlapping spots and two non-overlapping spots. The validity of each model was tested by comparing the ETV behaviour with the observed eclipse timings. The ETVs predicted by the overlapping and non-overlapping two-spot models both describe well the ETVs observed in the TESS data. In addition, the results from two-spot models indicate anti-correlated variations of spot sizes and spot longitudes throughout the TESS data time-span, whereas the longitudinal variations exhibit almost the same variation characteristics with the eclipse timings of the system. These findings imply that the longitudinal variation of spots predominantly affect the TESS-ETV behaviour of PP Lac. We conclude that the most likely explanation for light variation without eclipses and the TESS ETV behaviour in PP Lac is the presence of two high-latitude spots (or group of spots) located on the surface of the secondary component. With the help of the orbital stability tests and considering Applegate's mechanism, we found that the most plausible explanation for the two cyclic period variations seen on the overall ETV behaviour of the system is a combination of the magnetic activity cycle and the presence of a third body.